A torque converter is a hydrodynamic unit that transfers torque between an engine and an automatic transmission. The torque converter generally includes an impeller (driving member), a turbine (a driven member), and a stator that are disposed in a housing full of working fluid. The impeller is generally disposed at a rear portion of the housing (away from the engine), and it turns with a crankshaft of an engine. The turbine is generally disposed at a front portion of the housing (near the engine), and is connected to a transmission input shaft. The turbine is free to rotate independently from the impeller.
The working fluid flows from the impeller toward the turbine in a radial outer portion of the torque converter. The working fluid then flows from the turbine back toward the impeller by way of the stator in a radial inner portion of the torque converter.
As is well known in the art, in order to optimize efficiency it is necessary to select an appropriate torque converter for a particular engine configuration and application. For example, torque converters have traditionally been matched to a diesel engine by selecting a torque converter with a stall curve that crosses the engine torque curve approximately 100 rpm above peak engine torque, and a 0.8 speed ratio curve that crosses the engine torque curve at the engine governed speed.
For some engines such as, for example, highly turbocharged diesel engines, it may be necessary to match the stall curve of the converter with the engine's “lug up” curve. As will be described in more detail, a lug up curve is approximately the engines naturally aspirated torque curve up to the engine speed at which the turbocharger spins up. To properly match the stall curve of a torque converter with an engine's lug up curve, it may be necessary to provide a torque converter that is “looser” at stall than it is at 0.8 speed ratio. A “tighter” torque converter is one that can absorb more torque (has a higher capacity), and a “looser” torque converter is one that can absorb less torque (has a lower capacity).